(1) Field of the Invention
The present invention relates to a method and apparatus for the detection and diagnosis of an air-fuel ratio in a fuel supply control system of an internal combustion engine. More particularly, the present invention relates to a method and apparatus for diagnosing a disorder of air-fuel ratio-detecting unit for detecting an air-fuel ratio in an air-fuel mixture sucked in an engine based on the concentration of exhaust components in an exhaust gas from an engine in the feedback correction control for bringing the air-fuel ratio in the air-fuel mixture sucked in the engine close to a target value.
(2) Description of the Related Art
As the known fuel supply control system of an internal combustion engine, having a function of feedback control of an air-fuel ratio, the following system can be mentioned.
A sucked air flow quantity Q or sucked air pressure PB is detected as the quantity of the state participating in sucked air, and based on such detected values and the detected value of the engine revolution number N, the basic fuel supply quantity Tp is computed. Then, this basic fuel supply quantity Tp is corrected based on various correction coefficients COEF set by various driving state factors such as the engine temperature represented by the cooling water temperature, an air-fuel ratio feedback correction coefficient LAMBDA set based on the air-fuel ratio in the air-fuel mixture detected through the oxygen concentration in the exhaust gas and a correction proportion Ts by the battery voltage to compute a final fuel supply quantity (=Tp.times.COEF.times.LAMBDA+Ts), and fuel in the amount of this computed quantity is supplied to the engine through a fueL injection valve or the like (see Japanese Unexamined Patent publication No. 60-240840).
The air-fuel ratio feedback correction coefficient LAMBDA is set, for example, by proportional-integral control, and when the actual air-fuel ratio detected based on the oxygen concentration in the exhaust gas detected by an oxygen sensor is rich (or lean) as compared with the target air-fuel ratio (theoretical air-fuel ratio), the air-fuel ratio feedback correction coefficient LAMBDA is first decreased (or increased) by a proportion constant p and then gradually decreased (or increased) by an integration constant I synchronously or at the same frequency as that of the revolution of the engine. When the actual air-fuel ratio is brought close to the target air-fuel ratio, the changing direction of the air-fuel ratio feedback correction coefficient LAMBDA is reversed and this operation is repeated to effect the control.
As the oxygen sensor for the above-mentioned feedback control of the air-fuel ratio, there is generally used a sensor for detecting whether the actual air-fuel ratio is rich or lean as compared with the target air-fuel ratio by utilizing the phenomenon that the oxygen concentration in the exhaust gas abruptly changes with the target air-fuel ratio being the boundary. This oxygen sensor has a structure in which electrodes are formed on both of the inner and outer surfaces of a zirconia tube, an electromotive force corresponding to the ratio of the oxygen concentration in open air introduced into the inner side of the tube to the oxygen concentration in the exhaust gas to which the outer side of the tube is exposed is generated between the electrodes, and this electromotive force is monitored to indirectly detect the oxygen concentration in the exhaust gas and, in turn, detect whether the air-fuel ratio in the air-fuel mixture sucked in the engine is rich or lean as compared with the theoretical air-fuel ratio (see Japanese Unexamined Utility Model Publication No. 63-51273).
In the above-mentioned system of feedback control of the air-fuel ratio based on the results of the detection by the oxygen sensor, if the output characteristics of the detection signals to the air-fuel ratio are changed from the initial output characteristics by deterioration of the oxygen sensor, it becomes impossible to obtain the target air-fuel ratio (theoretical air-fuel ratio) with high degree of precision by the feedback control.
A ternary catalyst for purging the exhaust gas is often arranged in an exhaust system of an automobile engine. In this ternary catalyst device, a highest conversion efficiency is obtained when an air-fuel mixture is burnt at the theoretical air-fuel ratio. Accordingly, if the feedback-controlled air-fuel ratio deviates from the theoretical air-fuel ratio by the above-mentioned deterioration of the oxygen sensor, the conversion efficiency of the ternary catalyst device is degraded and there arises a problem of an increase of CO, HC and NO.sub.x. Furthermore, even in the case where the station characteristics of the oxygen sensor are hardly changed, if the response time of the oxygen sensor is changed from the initial response time when the actual air-fuel ratio is reversed from the rich state to the lean state or vice versa, the control point of the air-fuel ratio deviates from the initial control point (target air-fuel ratio), a problem arises in that a sufficient exhaust gas-purging effect cannot be attained by the ternary catalyst system.
As is apparent from the foregoing description, if deterioration of the oxygen sensor occurs, the feedback-controlled air-fuel ratio deviates from the theoretical air-fuel ratio and this deviation has an adverse influence on the properties of the exhaust gas. However, diagnosis of deterioration of the oxygen sensor is much more difficult than diagnosis of an on-off trouble of a single line such as a break or short circuit, and therefore, development of a diagnosis method or apparatus having high reliability is highly desirable.